Exhumation and carbonation of the Atlantis Bank core complex constrained by in situ U-Pb dating and Δ47 thermometry of calcite veins, SW Indian Ridge

Abstract

The Atlantis Bank is a representative example of an oceanic core complex formed on a slow-spreading ridge. It is dominated by olivine gabbros emplaced at ∼12 Ma and exhumed from original depths of more than ∼2500 metres below seafloor (mbsf). In order to investigate the timing of exhumation and low-temperature alteration we investigated twenty-two calcite veins in samples of core recovered from the 809-mbsf Hole U1473A drilled during Expedition 360 of the International Ocean Discovery Program. The calcite veins yielded laser ablation U-Pb and bulk U-Th ages ranging from 10.4±0.3 Ma to 273±5 Ka. They have an average Δ47 formation temperature of 21±4 °C and 87Sr/86Sr and rare earth element signatures typical of low-temperature veins dominated by seawater. These data preclude previous cooling models that invoked a 250 °C heat pulse related to sill emplacement at ∼9.4 Ma. Instead, the combined Δ47 and U-Pb age data indicate a simple cooling path in which the gabbros were exhumed close to the seafloor and cooled to a mean of 21±4 °C by 10.4±0.3 Ma. The timing of the earliest low-temperature veins 1.6±0.7 Myr after accretion is broadly similar to that reported for other oceanic core complexes. However, the 12 Ma age of the Atlantis Bank means it has recorded a longer carbonation history. The vein ages indicate ∼83% of vein growth occurred during a period of 3.1±0.9 Myr between ∼10.4 and ∼7.3 Ma. The Atlantis Bank gabbros are estimated to have a bulk concentration of 0.15±0.04 wt% CO2. Therefore if carbonation and veining occurred proportionally to one another, the initial carbonation rate was ∼400±200 μg/g CO2 per Myr. A single vein with an age of 5.8±0.4 Ma formed between 7.3 and 1 Ma suggesting a virtual hiatus in carbonation. However, resumed vein growth after 0.27-1 Ma is recorded by three samples. The uncertainty in these very young U-Pb ages is substantial, meaning the carbonation rate is only loosely constrained as similar to, or lower than, the initial rate of ∼400 μg/g CO2 per Myr. These data demonstrate that recent carbonation of 12 Ma old ocean crust is significant and probably ongoing. Given that ocean core complexes form on crustal-scale detachments that provide high permeability pathways, and they are bathymetric highs where reactive lithologies can be exposed on the seafloor, oceanic core complexes like the Atlantis Bank could be sites of long-lived alteration on the seafloor.